The developing nervous system undergoes a major transition in early postnatal life when it shifts from a period of exuberant synapse formation to a subsequent period of synaptic pruning and network consolidation. Synaptic numbers approach a maximum during this time, the rules governing synaptic plasticity change, AMPA and NMDA receptor compositions are altered, and GABAergic signaling shifts from being depolarizing/excitatory to hyperpolarizing/inhibitory, in addition to other changes. Mechanisms driving this profound transition are poorly understood at best. Interesting candidates for doing so are microRNAs (miRs) because they can target many different mRNAs at the same time for blockade or destruction, thereby producing a regulatory hub to coordinate complex changes across large systems. Our preliminary evidence indicates that miR-101 is an attractive candidate for executing this transition. It is abundant, increases substantially at the relevant time, and remains high in the adult. Using antagonists to block miR-101 function in vivo reveals significant aberrations: substantial increases are seen in spontaneous synchronized activity across neuronal populations and is accompanied by increases in synaptic number and excitatory synaptic input to neurons. Blocking miR-101 also appears to delay maturation of GABAergic signaling, enabling it to be depolarizing at later times. Importantly, our preliminary results using target site blockers to protect specific mRNAs suggest that miR-101 achieves its effects by acting on multiple targets. One is likely to be NKCC1, the chloride transporter responsible for GABA being depolarizing. But other miR-101 targets appear to be important as well, suggesting that terminating the depolarizing phase of GABAergic signaling is not by itself sufficient to account for all the major changes comprising the developmental transition. We will test the role of miR-101 in mediating the transition in nervous system development by using antagonists, sponges, and target site blockers to prevent its action in vivo while assessing the consequences for synaptic activity and network function. Calcium fluors in acute slices will report the frequency and extent of spontaneous coordinated activity across neuronal populations, as well as total activity and numbers of participating neurons. Patch-clamp recording will reveal type and amount of synaptic activity while immunostaining will quantify synapses. By selecting individual miR-101 targets for protection, it will be possible to dissect th contributions of different pathways to the transition and to evaluate the consequences of defective regulation. This will provide new insight into mechanisms driving the transition, reveal the role of miR-101 in particular, and likely have biomedical relevance by revealing the vulnerability of the system to individual regulatory pathways being compromised, producing, for example, greater propensity for epileptic seizure-like events.